The early satellite communication system designs employed an area coverage beam which provided interconnections on either a time-division multiple access (TDMA) basis or a frequency-division multiple access (FDMA) basis. Such designs had the disadvantage of low antenna gain and frequency reuse only by the use of polarization techniques. More recent designs use (a) multiple narrow-angle fixed spot beams with on-board satellite switching to provide frequency reuse, high capacity, and high antenna gain, (b) a single scanning beam to provide high antenna gain, (c) the combination of an area coverage beam and multiple narrow-angle fixed spot beams to provide high capacity, and (d) the combination of multiple narrow-angle fixed spot beams and a single scanning beam with on-board satellite switching.
In such systems, all transmissions from all ground stations are relayed through the satellite to the destined ground stations and any desired transmission format may be employed. In the TDMA systems, each of the stations transmit signals in the same frequency spectrum but in different time slots of a frame or superframe format so that the transmissions arrive in a preselected order at the satellite with no overlap. Signaling between ground stations has been accomplished by the use of separate telemetry channels in different frequency bands or by in-band signals in the preamble of the various bursts of information in each time slot.
A typical prior art technique used for in-band signaling is disclosed in U.S. Pat. No. 3,772,475 issued to A. Loffreda on Nov. 13, 1973. There, a superframe format with frame segmented signaling is used in a TDMA area coverage satellite communication system wherein destination signaling is time divided in the preambles of dedicated bursts of information throughout the superframe in a predetermined format.
Ranging and burst synchronization are also necessary in TDMA satellite communication systems to avoid overlapping transmissions arriving at the satellite. Coarse ranging of the distance of the satellite from each earth station can be accomplished by the well known technique of triangulation to provide coarse burst synchronization. Fine burst synchronization has been accomplished using various techniques. For example, in U.S. Pat. No. 3,639,838 issued to H. G. Kuhn et al on Feb. 1, 1972 a TDMA area coverage satellite communication system is disclosed wherein each ground station is allotted a number of sequential or randomly disposed time slots for transmission and, in addition, a frame synchronizing burst S during each frame.
In U.S. Pat. No. 3,754,102 issued to J. M. Clark on Aug. 21, 1973 a TDMA satellite communication system is disclosed wherein ground station transmissions are assigned in a superframe format which is divided into equal parts called "midframes" with each midframe comprising 15 subframes with odd subframes having an extra bit for transmitting signaling information and long and short sync code signals between ground stations.
The problem remaining in the prior art is to provide an in-band, two-way, signaling and ranging technique which avoids coupling signaling and ranging information bits to portions of a time slot also used for normally transmitting bursts of information of data between any two ground stations. By so coupling signaling information bits with information or data bits within a time slot burst, an inefficient format results since, if no traffic momentarily exists between two ground stations, the time slot must still be dedicated for possibly transmitting only the signaling information between the two stations.